Conventionally, there is proposed a technique for purifying an exhaust gas by using an electrically heated catalyst (hereinafter suitably referred to as “EHC”) provided on an exhaust passage. For example, in Patent Reference-1, there is proposed an EHC including a ring-like mat member formed by an electrical insulating material, which has a shock-absorbing characteristic and is positioned between an outer periphery of a catalyst carrier and an inner periphery of a metal shell. Additionally, in Patent Reference-2, there is proposed a technique for estimating a temperature of a seal mat which retains a catalyst.
Other than the above Patent References 1 and 2, there are disclosed a technique related to the present invention in Patent Reference-3, for example. In Patent Reference-3, there is proposed a technique for estimating a possibility of an electrical leak of the EHC based on a current or a voltage of the EHC, and for restricting a supply of an electric power to the EHC when there is a possibility of the electrical leak.
Hereinafter, a component which supports the catalyst in the EHC is referred to as “EHC carrier”, and a component which retains the EHC carrier is referred to as “retention mat”.